This invention relates to an improved method for recovering atactic polypropylene by-produced during the preparation of propylene homopolymer or a copolymer of propylene and one or more other .alpha.-olefins (both of which will hereinafter be referred to generically as "propylene-containing polymer").
In the course of the preparation of a propylene-containing polymer, atactic polypropylene is by-produced in a concentration of from 1 to 15% by weight based on that fraction of polypropylene which is useful as the desired product. Since the propylene-containing polymer product is insoluble in the polymerization solvent and non-polymerized .alpha.-olefin(s) used as the starting material, it remains suspended in the polymerization solvent and/or non-polymerized .alpha.-olefin at the end of the polymerization reaction. On the other hand, atactic polypropylene is soluble in the polymerization solvent or in the non-polymerized .alpha.-olefin at the end of the polymerization reaction, and therefore, the greater part of the atactic polypropylene formed is dissolved in the polymerization solvent and/or non-polymerized .alpha.-olefin, and the rest is mixed in the propylene-containing polymer product. However, the quality of the propylene-containing polymer product will be impaired if atactic polypropylene is contained therein beyond a certain limit. It is a common practice, therefore, to remove any atactic polypropylene from the propylene-containing polymer product by extraction with the .alpha.-olefin starting material, the polymerization solvent or an extractant.
Such atactic polypropylene may be used as a molding material (for the manufacture of carpet backings and the like), thickening agent, and solid fuel, or may be thermally decomposed to produce fuel oil. It is desirable in all of these applications that the residual volatiles content of atactic polypropylene be not greater than 5% by weight.
As stated above, atactic polypropylene is obtained as a homogeneous solution in a variety of solvents. For the above-described uses, therefore, it is necessary to recover solid atactic polypropylene by vaporizing the solvent. The method of recovery which has been commonly used in the prior art is based on the fact that atactic polypropylene goes through a sticky molten form during the recovery process. Specifically, an atactic polypropylene solution is preheated and fed to a thin film evaporator having a rotating shaft provided with scraper blades, where the solvent is vaporized while the atactic polypropylene is recovered by scraping sticky atactic polypropylene built up on the vessel wall. However, a thin film evaporator tends to develop troubles because of the mechanical complexity of its construction, besides being expensive.
Furthermore, as the vaporization of the solvent proceeds, the viscosity of the solution increases and the solvent becomes progressively more difficult to vaporize. Since a highly viscous melt is produced upon completion of the vaporization, resort must be made to mechanical drive means such as scraper blades or extruder screws, or else the melt will build up on the vessel wall and eventually clog the evaporation apparatus.